Electrodepositable compositions containing active hydrogen-containing ionic resins and capped polyisocyanate curing agents are well known in the art. For example, U.S. Pat. No. 4,031,050 discloses cationic resins which are prepared from reacting polyepoxide-containing polymers with primary and/or secondary amines and neutralizing the resulting reaction product with acid to form cationic salt group-containing polymers. The capped polyisocyanate curing agents for these polymers are polyisocyanates such as toluene diisocyanate (TDI) and 4,4'-diphenylmethane diisocyanate (MDI) alone or in admixture with other isomeric and analogous higher polyisocyanates (crude MDI).
To catalyze the curing reaction, an organotin catalyst such as dibutyltin oxide is usually present in the composition. Although effective as a catalyst, the dialkyltin compounds have certain shortcomings associated with them. They are solid materials at room temperature and are difficult to incorporate into the electrodepositable compositions. They are usually milled with a dispersing vehicle to form a catalyst paste and the paste added to the electrodepositable composition. The milling operation requires additional time, labor, equipment and adds to the cost of preparing the electrodepositable composition. Also, organotin compounds are expensive materials and somewhat toxic and can present environmental problems when used as a catalyst in the electrodeposition process. The materials can volatilize during curing and are often found in the ultrafiltration effluent. When attempts were made to minimize these problems by lowering the amount of dialkyltin catalyst in the electrocoating composition, the cured electrodeposited coating which resulted had poor cured coating properties and poor appearance.